This is something I've been really wanting for a while now. Unfortunately I don't know how complex this would be, but I'm hoping it will be simple enough for a coding snack I wouldn't be opposed to using AutoHotKey if it would be easier.

What I would really like is a small app that would let me click a mailto: link on one computer, and have it open a new email to that recipient on another system.

I use Stardock's Multiplicity; if you know about Synergy, then it's basically the same thing (a software KVM type solution). I keep all of my volatile data like email on a different machine than what I use for most of my browsing.

The system running the web browser would be XP SP3, and the email system would be Windows 7 x64.

The server-client app would be preferable, for sure (for all of the obvious benefits, including the ones you stated), but I would be fine with the client-only as well. With that said, I'll leave it to you and your schedule and motivation

kamahl, if you haven't started yet, I have a working, proof-of-concept application done. It's written using the client/server approach. Let me know if you still want dibs on this Snack because, c'mon, you can't deny a man his dibs.